Threaded joint with corrosion protection

ABSTRACT

A corrosion inhibiting system for protection of the threads of oil country tubular goods, including a vapor phase corrosion inhibitor and a liquid phase corrosion inhibitor in water solution, and a transparent thread protector. The threads may be inspected for corrosion without removing the thread protector or the corrosion inhibitor, and the corrosion inhibitor may be left on the threads for use in combination with an anaerobic sealing compound.

This is a continuation-in-part of copending application Ser. No.07/697,206 filed on May 8, 1991, now abandoned.

FIELD OF THE INVENTION

This invention relates to a method and system for protecting pipethreads from corrosion, and in particular is directed to such a methodand system for protection of oil field tubular goods, including tubing,drill pipe, casing and other apparatus having threaded connections. Theinvention includes the combination of a corrosion inhibiting compositionand a novel thread protector.

BACKGROUND OF THE INVENTION

A major industry has developed in the protection of the threads of oilfield tubular goods to prevent them from corroding during the periodbetween manufacture and use, because of the high cost of such tubulargoods and because of the importance of being sure that there are nodefects in the threads which could cause them to leak after the pipe hasbeen installed in a well. Oil well tubing often must withstand apressure of hundreds or even thousands of pounds per square inch. A leakafter the tubing is installed means the tubing must be pulled andrepaired or replaced, at a cost of perhaps hundreds of thousands ofdollars.

Pipe thread corrosion may be ordinary oxidation, or rust, or it may becaused or supplemented by micro-organisms which feed on variousmaterials on the surface of the thread and produce an acid which causespitting of the threads. A variety of means have been used to preventsuch corrosion, with questionable success. Commonly, API (AmericanPetroleum Institute) Bulletin 5A2 pipe dope has been used, although itis low in corrosion inhibiting properties. This pipe dope is designed tobe a thread compound, with lubricating and sealing properties. It is athick grease-based material which contains lead and other fillermaterials to seal the helical passageway in the round profile threadscommonly used on oil field tubular goods. Another material which hascome into use is a wax-based material sold under the trademark KENDEX.Other, lighter materials, such as a light oil, are sometimes used if thepipe is to be used within a day or two of the time it is threaded. Suchcorrosion inhibitors are applied to the threads as soon as the pipe isthreaded, in an effort to prevent flash rust which can occur in a veryshort time. Such flash rust is particularly objectionable on thehigh-precision threads known as "premium" threads which depend on ametal to metal seal.

After the corrosion inhibitor has been applied to a newly machinedthread, the thread is further protected by screwing on a cover known asa thread protector. Thread protectors may be made of metal or plastic,or combinations of metal and plastic, and are constructed to protect thethreads against impact damage when the pipe is accidentally dropped orbumped. Many thread protectors, aptly called "dust covers" in the field,are loose fitting and are of no value in keeping moisture away from thethreads. Some, however, are cup-shaped, i.e. they include an end coverto close the end of the pipe, and some are also snug fitting and includemoisture seals in an effort to improve corrosion protection.

In the manufacture of threaded tubular goods, the threading machines usea water-based cutting fluid. After threading, the cutting fluid iseither wiped off or blown off with air, the "corrosion inhibitor" isimmediately applied, and thread protectors are put on. The pipe is thenput in a pipe storage yard, usually exposed to the elements, until it isneeded. The pipe may remain in the storage yard for a year or more,depending upon demand. It is during this period that protection of thethreads against corrosion is most critical.

However, the oil, grease or wax-based materials which have previouslybeen used to inhibit corrosion have some major drawbacks. Themanufacturing operation, and even the methods used to clean the cuttingfluids from the threads, necessarily leave some water on the threads, aswell as some organic and sulfur compounds, and often microbes which canfeed on the organic and sulfur-containing materials and excrete acids.The grease and wax-based materials previously used do not absorb orremove the water, but instead enclose it on the surface of the threads,leaving oxygen and microbes in contact with the threads to do theirdamage.

Because of this, if the pipe, with corrosion inhibitor and threadprotectors in place, stays in a storage yard for more than about 60-90days, it is desirable to inspect the threads to be sure that corrosionhas not begun. This is done by removing the thread protectors, cleaningoff the corrosion inhibiting material with a solvent, steam or somemechanical means, inspecting the threads, reapplying corrosioninhibitor, and either applying new thread protectors or cleaning andreapplying the ones previously used. As long as the pipe is in the yard,this process should be repeated every 60-90 days to ensure againstthread corrosion.

This inspection process is expensive in the cost of the corrosioninhibitor and thread protectors, but more importantly in the cost ofmanpower for cleaning and retreating the threads. Moreover, every timethe lead-containing API dope is cleaned off, there is a contaminationproblem which must be dealt with. It has been estimated that as much astwo million pounds of lead are deposited on the ground each year throughthis process alone. Kendex and other inhibitors previously used arecleaned off with solvents, such as naphtha, diesel fuel or otherpetroleum-based materials which flow onto the ground and cause acontamination problem.

When the pipe is finally sent to a rig for installing in a well, it isnecessary to again remove the thread protectors, clean the threads,inspect for corrosion, apply a thread sealant, and then make up thejoints for installation in the well. Such materials as the API pipe dopeand Kendex are not compatible with the newer anaerobic sealants used asthread joint compounds, such as those disclosed in U.S. Pat. No.4,813,714, one embodiment of which is manufactured by LoctiteCorporation of Newington, Conn., and sold under the trademark SEALLUBE.It is necessary to remove the grease or wax based material andthoroughly clean the threads before this type of sealant is used.

Others have recognized the problem with the materials commonly used ascorrosion inhibitors on oil field tubular goods, and have proposedsolutions. Levesque, in an article entitled "Rust-inhibition fluidsprotect OCTGs in storage", published in WORLD OIL, March, 1985,discussed the use of tin or zinc plating or phosphate coatings for thisapplication. He also pointed out the desirability of non-metallic snugfitting thread protectors with moisture seals to reduce corrosion.

Vapor phase corrosion inhibitors are well-known for the protection ofsteel from corrosion during shipping and storage. Such inhibitors aredescribed in U.S. Pat. Nos. 2,643,177 and 3,779,818 and in theKirk-Othmer Encyclopedia of Chemical Technology, Vol. 7, pp. 137-138.These inhibitors produce a vapor which precipitates a very thin filmwhich is adsorbed on exposed surfaces. Known vapor phase inhibitorsinclude amine salts with nitrous or chromic acids, amine salts withcarbonic, carbamic, acetic and substituted or unsubstituted benzoicacids, organic esters of nitrous, phthalic or carbonic acids, primary,secondary and tertiary aliphatic amines, cycloaliphatic and aromaticamines, polymethylene amines, mixtures of nitrites with urea, urotropineand ethanolamines, nitrobenzene and 1-nitronaphthalene. Two common vaporphase inhibitors are dicyclohexylamine nitrite and cyclohexylaminecarbonate. However, both of these inhibitors have some degree oftoxicity, and various non-toxic proprietary compositions have thereforebeen produced and are in use to provide corrosion protection. One ofthose compositions is sold under the trademark Cortrol VCI, and ismanufactured by Corless North-America, Inc. of Stanford, Conn. CortrolVCI is a water soluble mixture of sodium benzoate and approximately 15%to 25% alkylated sodium benzoate, containing trace amounts of water andamine salts. Such a material may be made by the partial oxidation ofcoal tar distillates which contain mixtures of toluene, xylene and otheralkylated benzenes to form benzoic acid and alkylated benzoic acid,followed by reaction with sodium hydroxide or sodium carbonate.

The vapor phase inhibitors are intended to be used dry, but many of themwill continue to provide some corrosion control when dissolved in water.Generally, however, when the surface to be protected is immersed inwater, as in steel tanks or boiler tubes, liquid phase inhibitors, suchas the water soluble inorganic and organic salts which create a passivesurface on the metal, are used. Such liquid phase corrosion inhibitorsare disclosed, for example, in U.S. Pat. No. 2,550,997, and include thenitrite salts of alkali metals, alkaline earth metals and aromatic,aliphatic and heterocyclic amines which are not subject toauto-decomposition at ambient temperature. Benzoic acid and its salts,such as sodium benzoate, potassium benzoate and ammonium benzoate, arealso effective liquid phase inhibitors. Because of cost factors andavailability, sodium benzoate, sodium nitrite and potassium nitrite aregenerally preferred.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method and system forprotection of pipe threads from corrosion which eliminates the problemof contamination from removing corrosion inhibiting materials.

Another object of the invention is to provide a thread protection systemwhich is environmentally safe, can be used without cleaning the pipethread before making up joints, and which is less costly than systemspreviously used.

It is a further object of the invention to provide a thread protectingmethod and system which includes a combination of a thread protector anda corrosion inhibitor which allows inspection of threads for corrosionwithout disassembling the thread protector from the pipe and withoutcleaning the corrosion inhibitor from the threads.

Another object of the invention is to provide a transparent threadprotector which allows inspection of pipe threads without removing theprotector, and which has sufficient toughness and impact resistance toprevent damage to pipe threads if the pipe is accidentally dropped onthe threads.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the drawing depicts a longitudinal sectional view of a tubingend and one embodiment of a thread protector of the invention,

FIG. 2 depicts a fragment of a threaded connection between male andfemale threads using one embodiment of the invention, and

FIG. 3 shows an enlarged section of the threaded connection of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to this invention, pipe threads are protected from corrosionby applying to the threads an inhibitor which will not conceal corrosionof the threads, i.e. the inhibitor is transparent to corrosion.Preferably, a transparent plastic thread protector, i.e. one whichallows visual inspection of the threads with the protector installed, isthen screwed onto the threads. With this combination of elements, thethreads may be periodically inspected without removing the threadprotectors to determine whether any corrosion has begun. Any corrosionwill be visible through the transparent thread protector and theinhibitor coating, so it is not necessary to remove the inhibitor fromthe pipe threads to inspect them. Thus, there is no contamination of thepremises when this procedure is followed.

The corrosion prevention method and system of this invention preferablyuses a unique inhibitor comprising a combination of a liquid phaseinhibitor and a vapor phase inhibitor dissolved in water. The preferredinhibitor solution contains no grease or oil, but in fact includes asurfactant to facilitate close contact of the water based inhibitorsolution with the pipe thread to be protected. However, other corrosioninhibitors which do not include components which will adversely affectthe material of the thread protector, and which are effective inhibitorsin the form of coatings which are transparent enough to allow inspectionof threads for corrosion without removing the transparent threadprotector, may also be used. For example, with some thread protectormaterials, a thin film of a light oil may be used. It is also preferredthat such other inhibitors be compatible with the thread compound orsealant which will be used to make up the pipe when it is put into thewell, so that it is not necessary to remove the inhibitor from the pipethreads before applying the thread sealant.

According to this invention the inhibitor used is transparent tocorrosion, i.e. it is thin enough, or transparent enough, that corrosionof the pipe threads coated with the inhibitor is visible withoutremoving the inhibitor, or the inhibitor changes color when the threadscorrode. The preferred inhibitors of this invention are those which donot interfere with the effectiveness of the thread compound used tolubricate and seal the assembled pipe joints. The preferred inhibitorsare most advantageously used on pipe threads which are to be assembledwith anaerobic thread compounds, such as the product sold under thetrademark SEALLUBE, which polymerize in the absence of air to form hard,durable resins, but these inhibitors are also preferred when otherthread compounds are used because of the many advantages of suchinhibitors. The preferred inhibitors are environmentally safe, and theiruse saves a tremendous amount of labor and material cost, because it isnot necessary to remove the inhibitor to inspect the pipe threads forcorrosion. Furthermore, they are less expensive than other materialsthat are available.

The preferred inhibitors of this invention include a vapor phaseinhibitor comprising a salt, preferably a sodium, potassium or ammoniumsalt, of benzoic acid and its alkylated homologs, such as may beproduced by the oxidation of aromatic distillates of coal tar consistingprimarily of toluene and other lower boiling point distillates. Thesodium salt is preferred, because of its cost and availability, and itslack of toxicity. The inhibitor may consist primarily of sodiumbenzoate, but a substantial proportion of alkylated material, e.g. fromabout 5% to about 50% or more, is preferred. Such a product is soldunder the trademark Cortrol VCI, by Corless North-America, Inc., and isknown as a vapor phase inhibitor. Cortrol VCI contains from about 15% toabout 25% of the alkylated material. In the process of this invention,the vapor phase inhibitor is applied in water solution, although it isan effective inhibitor as a solid.

Preferably, the vapor phase inhibitor is combined with a liquid phaseinhibitor, a surfactant and, in the preferred embodiment, a mono- orpoly-hydroxyl alcohol to lower the freezing point, all dissolved inwater. The preferred liquid phase inhibitor for use with the vapor phaseinhibitors described in the preceding paragraph is sodium benzoate.

A preferred composition is prepared by initially preparing a saturatedsolution of sodium benzoate in water, adding the surfactant, thendissolving Cortrol VCI in the solution until saturation is reached. Thealcohol may be added to the solution at any convenient time. A suitablesolution of the inhibitor of this invention may be made by preparing asaturated solution of sodium benzoate in water, requiring approximately150 pounds of sodium benzoate in 36 gallons of water, adding a smallamount, from about 1/10 pound to about pound, of a liquid surfactant andthen dissolving sufficient Cortrol VCI to create a saturated solution,about 55 pounds. Finally, about 30 pounds of propylene glycol are added.

A saturated, or nearly saturated, solution is preferred for general use,because of the reduced volume of material required, the more effectivecorrosion prevention obtained, and reduced cost of storage andtransportation. However, this inhibitor is effective in very smallamounts, so that in some applications it may be desirable to use a muchlower concentration of the liquid phase and the vapor phase inhibitors,for example as low as 2% to 10% by weight of each material. A slightlysupersaturated solution may also be prepared, but this may result insome of the material precipitating out of solution in cold weather. Inany case, it is desirable to have sufficient alcohol in the solution tominimize possible precipitation of solids, and to prevent freezing atambient temperatures. If the material is to be used at temperaturesabove about five degrees Celsius, the alcohol may be omitted Thus fromzero to about 10% by weight of alcohol may be used. The amount ofsurfactant is not critical, the purpose being to improve the ability ofthe solution to form a film on the threads, and to provide sufficientsurfactant to cut any oil or grease film on the threads so the inhibitorcan achieve a good contact with the threads. Thus, as little as 0.1% ofsurfactant ma be used.

Although the inhibitor composition just described is preferred, theinvention is not limited to this particular composition, but extends toall inhibitors and combinations of inhibitors which will preventcorrosion of the threads for an extended period of time, such as up toone year, when exposed to the weather and enclosed within a threadprotector, and which have the requisite transparency to allow inspectionof the threads for corrosion without removing the inhibitor. Thepreferred inhibitors also function whether wet or dry. In particular,such preferred inhibitors include a combination of a vapor phaseinhibitor and a liquid phase inhibitor, preferably both water soluble.

When a "dust cover" protector, i.e. one which does not fully seal in thethreads or cover the end of the pipe, is used, the water in theinhibitor solution will usually evaporate, leaving a thin, white filmdeposited on the threads, the film containing both the vapor phaseinhibitor and the liquid phase inhibitor. When a premium protector, i.e.one which completely encloses the threads and the end of the pipe, isused, the water base may not evaporate completely; nevertheless, thethreads are protected by the synergistic effect of the two inhibitors.

The preferred inhibitor of this invention is effective even if thethread protector leaks a little and allows some moisture to contact thethreads, as for example when it rains. The moisture will merelyre-dissolve the inhibitor which is on the threads, so that the liquidphase inhibitor continues to provide protection.

The preferred inhibitor composition of this invention has a high pH,e.g. 8 or more, so it serves to neutralize any acid which may be on thethreads or which may result from microbial action. In addition, theinhibitor contains no sustenance for microbes, so it is antimicrobial.

In use, the preferred inhibitor composition of this invention is usuallysprayed or brushed on the threads 12 of the tubing 10 as shown in FIG. 1of the drawing. Because it is water based, the inhibitor combines withany water that is present on the threads. Grease-based compoundspreviously used hold the oxygen-containing water against the threads andcause pitting and rusting damage. The inhibitor of this invention workswet or dry. The vapor phase inhibitor produces a monomolecular film atthe surface of the metal which migrates to all recessed areas andcavities and binds to the outer valence molecule of the iron compound sothat the iron acts as though it were inert to the oxygen molecule. Theinhibitor dries to a thin white film. After the inhibitor is applied tothe clean threads, a thread protector 14 is screwed on. The threadprotector is preferably cup-shaped, i.e. with a closed end, as shown inthe drawing.

Preferably the thread protector should be made of an impact resistanttransparent material which has as least some resistance to ultra-violetdegradation which undesirably affects transparency or toughness. It isimportant that the material used have sufficient transparency to permitviewing changes in the color of the enclosed threads, such as occurswhen the steel corrodes, and that such transparency can be maintainedfor at least six months while the pipe using the protectors is storedoutdoors. Suitable materials include styrene-butadiene copolymers,polycarbonates such as General Electric Company's Lexan 113, acrylicssuch as Rohm & Haas Company's Plexiglass, high density polyethylene,clarified polypropylene, polyvinyl chloride and the like. Satisfactoryresults may usually be obtained from materials which have a luminoustransmittance of at least about 85% and a haze value of not more thanabout 5%, both as determined by ASTM Standard D1003. However, under somecircumstances other materials will allow adequate viewing of the threadsto achieve the objectives of the invention. If the material is one whosetransparency is adversely affected by ultra-violet within a short time,an antioxidant may be included in the composition. Such materials arewell known in the art, and those skilled in the art can readily selectfrom those available. One suitable compound is sold by Phillips 66Company under the name KR03, and comprises a copolymer of three partsstyrene and one part butadiene with an anti-oxidant. This material has atotal luminous transmittance of 89-91% and a haze value of 1 to 3%, bothas determined by ASTM Standard D1003. The thread protectors can be madefrom recycled materials such as milk cartons and soft drink bottles. Apreferred thread protector has a small amount, e.g. 5% to 10%, of vaporphase inhibitor incorporated in the polymeric material to add to theeffectiveness of the inhibitor which has been applied to the threads. Insome applications, this inhibitor alone may provide sufficient corrosionprotection.

The thread protector may be molded, as by injection molding, ormachined, or partially molded and partially machined. Because of thelarge number of different threads that must be provided for, and thecost of molds, it has been found to be economically advantageous toinjection mold the part without threads, and then machine a threadprofile to fit the desired application.

Such transparent thread protectors are also useful with other inhibitorswhich do not conceal the threads, such as light oils, and variouscommercial rust inhibitors such as that sold under the trademark WD-40.The butadiene-styrene copolymers, however, are adversely affected bysuch light oils.

It will be apparent that the material used for the thread protectorshould be one which is not so brittle that it is easily broken. Thethread protector must have sufficient strength and impact resistance "toprotect the thread and pipe end from damage under normal handling andtransportation". American Petroleum Institute Specification 5CT, Section7.5. To protect the thread and pipe end from damage when the pipe isaccidentally bumped or dropped, greater strength and impact resistanceare required. These characteristics of a protector for oil field tubingcan be insured by a test in which a pipe nipple with a protectorinstalled is placed on a support with the protector end angled upwardlyat 45 degrees. A weight equal to the weight of a joint of pipe (a 30foot length) is dropped on the protector from a height of two feet. Thisimpact should not deform the end of the thread sufficient to make itmore than 0.005 inch out-of-round. With oil field casing, which is muchheavier than tubing, the drop height chosen is that height which willcause 0.005 inch out-of-round on unprotected pipe thread. Preferably,the protector is tough enough that it will not shatter under thesetests.

Advantageously, the thread protector may be provided with a port 16fitted with a grease fitting 18 through which additional inhibitor maybe applied from time to time during storage if inspection reveals lossor deterioration of the originally applied inhibitor. Since the threadprotector is transparent, one may readily determine whether theinhibitor has penetrated to all the threaded area of the tubing.

Although the drawing shows a male pipe thread, female pipe threads maybe similarly treated with the inhibitor of this invention and providedwith a transparent thread protector.

To be sure that the inhibitor is working, periodically the threadsshould be inspected to make sure that they are not corroding. With thetransparent thread protector, a visual inspection is possible withoutremoving the thread protector. The preferred inhibitor of this inventiondries to a thin white film, and any corrosion which may occur under thisfilm will be apparent because the film is only a few molecules thick.Thus it is possible to inspect the threads for corrosion withoutremoving the transparent pipe thread protectors.

When the pipe is ready for use, the thread protector is removed and asuitable thread compound or sealant is applied. Since the preferredinhibitor compositions are compatible with thread compounds, it is notnecessary to clean the inhibitor from the threads before applying thethread compound. As shown in FIG. 2, when a connection is made up, thereis a small space 22 between the mating threads 24, 26. The width of thespace 22 is exaggerated in the drawing to more clearly show thematerials in the space. As seen in FIG. 3, the inhibitor appears as athin film 28, 30 on the surfaces of the threads, with the sealant 32filling the space 22.

The transparent thread protectors of this invention have otherbeneficial uses, particularly in application to oil field tubular goods.For example, a corrosion warning indicator can be applied to the threadsto indicate when the threads have been exposed to a condition which willlikely lead to corrosion. Such a condition may arise when a load of pipeis being transported by barge to an offshore location, and is splashedwith sea water. An indicator of an acid condition on the threads willshow when enough sea water has entered the thread protector to create anacid condition. A reagent such as phenolphthalein or methyl orange willproduce such an indication.

Several embodiments of the invention are disclosed, however theinvention is not limited to these specific embodiments, but includes allvariations included within the scope of the claims.

I claim:
 1. A method for protecting matching male and female threads onoil field tubular goods from corrosion from the time the threads areoriginally machined until the tubular goods are installed in a well,comprisingapplying to each of the male and female threads, immediatelyafter the threads are machined, a corrosion inhibitor which istransparent to rust on the threads, storing the threaded tubular goodsfor later use in an oil well, thereafter, prior to making up the threadsfor use in the oil well, and without removing the corrosion inhibitorfrom the threads, applying to the threads a sealant which is notincompatible with the corrosion inhibitor, and making up the male andfemale threads.
 2. A method as defined by claim 1, wherein the sealantis an anaerobic material which polymerizes in the absence of air.